When looking at some costumes, I'd strongly consider getting them- the entire costume looks great! Except... EXCEPT... the hideous shoulder-pads. And the backpack on the treasure hunter costume is pretty awesome- it's a pity it can't toggle on and off. Oh! What if there was a backpack you could wear over ANY costume? Or traveler's cloak? A fishing pole!?
Here are my suggestions:
1. Ability to Hide Shoulders, like we can Hide Helm
I want to be able to hide shoulders. This would make many more costumes more attractive to buy for me, and I'm sure for others as well. There's no reason for an otherwise good-looking set of leathers to have horrific, weirdly shaped shoulders that detract from the entire thing.
2. A costume slot for your back
Backpack, traveling cloaks (different from capes), fishing pole, other various tools. It would be awesome to see a slot for these in addition to all the other new ones being added for head, face, etc. Because it would be a costume, nobody has to worry about re-balancing armor to fit in a back slot- this is just for fun! And once again, makes money.
Bonus suggestion: Werewolf polymorph
If I can't have toggle-wolf (with appropriate guard against exploiting ultimates, such as the second you toggle it you lose all your built up ultimate, but you can stay in that form indefinitely), I would really like a werewolf costume. Please. Pleeeeeeeeeeease.